1. Field of the Invention
The present invention relates to a large-diameter optical fiber preform fabricating method, and in particular, to apparatus and method for overcladding an optical fiber preform rod, which reduces a preform fabrication time by preheating a glass tube with a furnace, heating the glass tube with an oxygen-hydrogen burner, and collapsing the glass tube onto the preform rod, and an optical fiber drawing apparatus and method which feeds a glass tube and preform rod into a furnace that applies heat to collapse the glass tube on the preform rod to form a preform and that applies heat to the preform in order to draw an optical fiber from the furnace.
2. Description of the Related Art
An optical fiber is generally fabricated in two processes: a preform rod is prepared and then a preform is fabricated by a rod-in-tube or overcladding method, in the first process; and an optical fiber having an outer diameter of 125 .mu.m is fusion-drawn from the fabricated preform, in the second process.
The preform rod is fabricated by outside deposition or inside deposition. In the outside deposition process such as a VAD (Vapor Phase Axial Deposition) process and an OVD (Outside Vapor Deposition) process, SiO.sub.2 particles called soot are deposited onto a preform rod from the outside by hydrolyzing chemical gases such as SiCl.sub.4 and other dopants together with oxygen by flames, while supplying the gases to the preform rod. Then, this porous preform rod is fed into a furnace, dehydrated, and sintered, using Cl.sub.2 and He, thereby obtaining a transparent preform rod. On the other hand, in the inside deposition process including a CVD (Chemical Vapor Deposition) process and a MCVD (Modified CVD) process, a plurality of layers are deposited on the inner surface of a glass tube by providing SiCl.sub.2 and other dopants together with O.sub.2 into the tube, and the layer-deposited glass tube is heated at a high temperature and collapsed, while supplying Cl.sub.2 and He into the tube. Thus, a glass rod is obtained. The MCVD process is widely used and enables fabrication of a high-quality glass preform rod.
The MCVD and CVD processes for fabrication of an optical fiber are limited in obtaining a preform rod having a diameter of about 23 mm or above in view of their processing characteristics. Hence, to increase product yield, an overcladding method has been explored in which a glass tube is fusion-stuck to a preform rod prepared by the above inside deposition processes.
To obtain a large-diameter preform, a prepared preform rod is inserted into a large-diameter glass tube, which is then heated and collapsed onto the rod in the rod-in-tube or overcladding method, as is well known and thus will not be described in detail. This is disclosed in detail in U.S. patent application Ser. No. 08/292,977 entitled Single-Mode Primary Overcladding Method and Apparatus, and U.S. Pat. No. 4,820,322 to Jerry W. Baumgart et al and entitled Method And Apparatus For Overcladding A Glass Rod which utilizes a vertical lathe and a rod-in-tube, or overcladding, method to obtain a large-diameter preform, wherein a preform rod is inserted into a large-diameter glass tube, heated, and collapsed onto the glass tube while the pressure in the gap between the tube and rod is reduced by a vacuum. Another method described therein uses a zirconia induction furnace to collapse a tube on a rod during an optical fiber drawing operation.
Though there is no difficulty in inserting and overcladding a preform rod fabricated by the MCVD process into and with a glass tube having an outer diameter of 70 mm, the amount of heat required for overcladding increases as the outer diameter or thickness of the glass increases and, as a result, the overcladding rate of a burner for externally providing heat should slow down. The problem may be overcome by farther lowering a vacuum pressure applied to the interface between the preform rod and the glass tube, but a very large negative pressure brings about degradation of concentricity and circularity in the cross section of a preform.
On the other hand, the heat energy provided from outside can be increased simply by increasing the supply flow rate of a current oxygen-hydrogen burner. The outer surface of the glass tube is softened, however, resulting in a lower viscosity, while the inner surface thereof is rather slowly softened, keeping a predetermined viscosity. Therefore, the surface of the glass tube may be deformed by the flame pressure of the oxygen-hydrogen burner at the increased supply flow rate, or contaminating particles may adhere from the burner to the surface of the large-diameter glass tube. The oxygen-hydrogen burner cannot transfer heat sufficiently to the surface of the glass tube due to its relatively short hot zone, and brings about a non-uniform temperature distribution on the periphery of the glass tube. Hence, geometric irregularities such as ovality in the cross section of the glass tube occur, and the difference between the viscosities of the outer and inner surfaces of the glass tube increases microbending loss. Moreover, product yield is remarkably lowered because about 2-4 hours is required to fabricate a preform.
From the preform fabricated in the overcladding method, an optical fiber having an outer diameter of 125 .mu.m is fusion-drawn at a predetermined linear velocity under a predetermined tension load. The keypoint of the drawing process is to increase productivity per unit time by increasing the linear velocity, and a current linear velocity is usually 600-1200 m/min.
However, the above optical fiber drawing method has distinct drawbacks in that mass production of optical fibers is impossible due to the low linear velocity, and product yield decreases and optical fiber cost increases by adding the overcladding process for fabrication a preform from a preform rod before drawing an optical fiber.
Other known apparatus and processes for fabricating optical fiber preforms and drawing optical fibers therefrom are discussed in U.S. Pat. No. 2,980,957 to J. W. Hicks, Jr. and entitled Method And Apparatus For Use In The Fabrication Of Light-Conducting Devices which describes an apparatus and method for collapsing a glass tube on a glass rod utilizing a vertical holding apparatus, wherein gases between the tube and rod are drawn out by a vacuum. The formed preform is than drawn out from the apparatus to form an optical fiber. U.S. Pat. No. 4,602,926 to Andrew P. Harrison et al. and entitled Optical Fibre Fabrication which describes a method for manufacturing an optical fiber by feeding a rod and a tube into a furnace at different rates and the diameter of the fiber drawn from the furnace is monitored. U.S. Pat. No. 4,793,842 to Hiroshi Yokota et al. and entitled Method For Producing Glass Preform For Optical Fiber which describes a method for collapsing a tube on a rod to manufacture a preform, wherein the gap between the tube and rod is filled with a gaseous mixture of silicon halogenide, a fluorine-containing compound and oxygen and preheated at a temperature between 500 and 1900 degrees Celsius. After the preheating step the atmosphere in the gap is replace by a gaseous mixture of a halogen-containing compound and oxygen, one end of the tube is collapsed on the rod to form a seal and then the tube is collapsed on the rod while decreasing the pressure in the gap by means of an exhausting apparatus. And European Patent No. 501429-A1 to Masami Ito et al. and entitled Method For Producing Glass Preform Form Optical Fiber describes a process of attaching a tube and a rod to a vertical lathe, inserting the rod into the tub, filling the gap between the tube and rod with a halogen-containing gas and oxygen gas, then collapsing the tube on the rod to form an optical fiber preform.